1. Field of the Invention
The present invention is directed to a method and apparatus for dynamically controlling the generation of x-ray pulses during fluoroscopic imaging. More particularly, the present invention is directed to an apparatus and method for controlling the x-ray pulse frequency during fluoroscopic imaging to compensate for motion and image brightness, reduce radiation dosages and operator exposure and hasten image stabilization.
2. Description of the Related Art
In a conventional x-ray fluoroscopy apparatus, an x-ray source transmits a continuous beam of x-rays through a mass or body, such as a patient. An image intensifier is positioned in the path of the beam opposite the source with respect to the body. The image intensifier receives the emerging radiation pattern from the body (the detected dose) and converts it to a small, brightened visible image at an output face thereof. The output image of the image intensifier is viewed by a television camera and produces a dynamic real time visual image, which can be displayed on a CRT for interpretation or viewing by a doctor or operator and/or can be recorded. The resulting two dimensional image can be used for diagnosing structural abnormalities within the body.
X-rays are absorbed by regions in the body in varying degrees depending upon the thickness and composition of the regions. Accordingly, the ability to see structure in the body using fluoroscopy depends upon the x-ray absorption properties of the structure of interest in the body relative to the x-ray absorption properties of the structures adjacent to the structure of interest. When the difference in absorption between such structures is greater, the greater the contrast and the greater the clarity of the structure. In this regard, a great deal of effort has been put forth to obtain the maximum contrast possible. In one technique, radiographic contrast agents are introduced to a body to provide a difference in x-ray absorption properties where none or little previously existed, such as between soft tissues and blood vessels. For example, a bolus containing iodine or barium, which have x-ray absorption characteristics different than blood, muscle and soft tissues in general, can be introduced into an artery or vein to provide the vascular system with a greater contrast in a certain vascular segment. Digital image processing techniques are also employed to increase contrast. For example, in image subtraction, a field of interest is imaged sequentially using x-ray beams of different energy levels, or using constant energy levels in combination with contrast agents to provide images before the agent has reached the field of interest and then after the agent has arrived. The corresponding images are then digitally subtracted from each other to maximize contrast.
In addition to contrast, the detected dose and motion of or within the body are two factors which affect image quality. The brightness of an image produced by a fluoroscopic system, for example, is directly dependent upon the detected dosage. The detected dose is dependent upon the absorption of x-rays in the field of interest and the strength of the x-ray beam output from the x-ray source. Factors which affect the detected dose for a given diagnostic procedure include the characteristics of the structures within the field of view, the size and weight of the patient and the strength of the x-ray beam. As these factors can vary widely from patient, systems which compensate for these factors have become desirable.
Fluoroscopic systems first developed employed a continuous x-ray beam. In these systems, the strength of the x-ray beam could often be preset to a level deemed appropriate by the operator dependent upon the patient and the procedure. Improved systems were able to automatically adjust the brightness of images produced therefrom by automatically adjusting the strength of the x-ray beam. One such technique involves automatically adjusting the kilovoltage (kV) applied to the anode of the x-ray tube to maintain optimal brightness of the image. Typically, when a decrease in brightness is detected, the kV is increased to increase the x-ray output and hence increase the brightness of the output image, and, conversely, when an increase in brightness is sensed, the kV applied to the anode of the x-ray source is reduced to decrease the x-ray output and subsequently decrease the brightness of the output image. For example, such systems are disclosed or discussed in U.S. Pat. No. 4,703,496 to Meccariello et al. and 4,910,592 to Shroy, Jr. et al.
More recently, systems have been introduced which also adjust the x-ray tube milliamperage (mA) to keep the image brightness constant. Such systems, for example, adjust the x-ray tube photon output by adjusting the level of current (mA) used to heat the filament. However, such systems do not stabilize quickly due to the time required to increase or decrease photon intensity when adjusting the mA, thereby extending the period during which the patient is exposed to radiation. Additionally, by increasing the mA, the patient is exposed to more radiation.
However, problems exist with these systems. Maximum permissible x-ray doses to patients are mandated by health and government organizations. Due to these dosage limitations, the brightness stabilization techniques of these systems cannot always compensate for a decrease in image brightness. Additionally, when the brightness is being adjusted, the image is subject to excessive flicker, and image stabilization time is relatively large. Further, due to an inherent lag in such systems, image smearing is common when motion occurs in the field of view. Image smearing obscures portions of the image and can cause a doctor or operator to miss valuable image information.
Both the Meccariello and Shroy, Jr. patents attempt to resolve the brightness problem at least in part by some kind of television camera gain control. However, when gain is increased, noise is amplified as much as picture information, and no additional information results because the picture information is limited by the strength of the x-ray beam being input to the body. Still, the transition when gain is increased or decreased is not smooth, causing noticeable flickering when increasing and decreasing brightness and requiring time to stabilize the image.
Another problem with all these prior systems is that even though the x-ray dosages may be within limits, they tend to solve brightness problems in part by increasing the amount of x-rays employed. Additionally, long stabilization times in view of motion or a need to adjust for brightness tend to be inherent in the prior systems. While these systems typically do not exceed the maximum prescribed patients dosages, patients are nonetheless exposed to increased radiation levels during periods of adjustment. In view of recent growing concerns regarding exactly how much, if any, exposure to radiation is "safe", limiting exposure levels is becoming a concern in the industry. Possibly of more concern is the amount of x-rays to which an operator will be exposed. However, reducing radiation exposure of patients and operators is simply not adequately addressed by many prior systems.
A relatively recent development which does reduce radiation exposure is pulse progressive fluoroscopy. In pulse progressive fluoroscopy, individual x-ray pulses are generated at what is typically a predetermined rate, and each pulse is converted into an image for viewing until the next pulse is received. While the patient is exposed to less radiation, problems associated with motion and changes in the detected dose are more severe. Stabilization times are extremely long when motion occurs or the detected dose changes.
Given the long stabilization times in the prior art systems, valuable doctor time is wasted and energy requirements for the systems are high. And in this era of ever escalating health care costs, such considerations cannot be taken lightly.
The problems identified above are magnified in view of motion, either by the patient or by the subject of interest within the patient, such as the heart. Some of the prior systems acknowledge this problem and provide some image improvement in view of motion, but at the expense of the problems and drawbacks identified above.
Clearly, a need exists for a fluoroscopic imaging system which provides fast stabilization, lower dosages to patients, and decreased operator exposure when adjustments in response to a change in the detected dose and/or motion in or of the field of interest are being made.